Same problem here. Alfa 156 2.0 16V EURO3.
In factory line, at 90C, tehnician mesures NTC resistance at sensor screwed in thermostat case, and then it picks up stable resistance part and weld it to temperature gauge. Inside, there is a "bridge" system. It works this way: When two resistance in bridge are equal, gauge pin hits middle of the scale (90C).
1. You must find independent way to mesure temperature (like red point laser temperature digital sensor), and when it reaches 90, put the ohmmetter in sensor, and find out how may ohms are there. Write it down.
2. Open temperature gauge, and you will find big ressistor. Read it's value. If it does not match, find another one who matches your new sensor. And, that's it.
I hope you understand it, now.
***do not use OBD to messure temperature, it will lie, for same reason. find independent temperature sensor with scale.